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NIR Transmission Gratings![]()
GTI50-03A (50 mm x 50 mm) GTI25-03A (25 mm x 25 mm) ![]() Please Wait
Features
Thorlabs' Near Infrared Transmission Gratings are designed for the 500 nm to 1.8 μm range. Due to their basic simplicity, transmission gratings are beneficial for use in fixed grating applications, such as spectrographs. The incident light is dispersed on the opposite side of the grating at a fixed angle. Transmission gratings provide low alignment sensitivity, which minimizes alignment errors. These grooved transmission gratings were designed for optimum performance in the near infrared, offering different levels of dispersion. In most cases, the efficiency of these gratings is comparable to that of reflection gratings such as Ruled Gratings or Holographic Gratings when used in the same wavelength range. By necessity, transmission gratings require relatively coarse groove spacings to maintain high efficiency. As the diffraction angles increase with the finer spacings, the refractive properties of the substrate materials used limit the transmission at the higher wavelengths and performance drops off. The grating dispersion characteristics, however, lend themselves to compact systems utilizing small detector arrays. The gratings are also relatively polarization insensitive. Our NIR transmission gratings are offered in two different sizes, with a choice of two groove angles. Please see the graph to the right for performance characteristics. Please see the Gratings Guide tab to choose the right grating for your application. Mounts and AdaptersThorlabs' gratings can be mounted directly into the KM100C Right-Handed or KM100CL Left-Handed Kinematic Rectangular Optic Mount for precise and stable mounting and alignment. Warning:Optical gratings can be easily damaged by moisture, fingerprints, aerosols, or the slightest contact with any abrasive material. Gratings should only be handled when necessary and always held by the sides. Latex gloves or a similar protective covering should be worn to prevent oil from fingers from reaching the grating surface. No attempt should be made to clean a grating other than blowing off dust with clean, dry air or nitrogen. Solvents will likely damage the grating's surface. Thorlabs uses a clean room facility for assembly of gratings into mechanical setups. If your application requires integrating the grating into a sub-assembly or a setup, please contact us to learn more about our assembly capabilities. The absolute efficiency plotted here includes Fresnel reflections. ![]() Click to Enlarge Top Image: On one edge of the grating, an arrow parallel to the grating's surface indicates the blaze direction. Bottom Image: On the opposite edge of the grating, an arrow perpendicular to the grating's surface indicates the transmission direction. Diffraction Gratings TutorialDiffraction gratings, either transmissive or reflective, can separate different wavelengths of light using a repetitive structure embedded within the grating. The structure affects the amplitude and/or phase of the incident wave, causing interference in the output wave. In the transmissive case, the repetitive structure can be thought of as many tightly spaced, thin slits. Solving for the irradiance as a function wavelength and position of this multi-slit situation, we get a general expression that can be applied to all diffractive gratings when ![]() (1) known as the grating equation. The equation states that a diffraction grating with spacing
Figure 1. Transmission Grating Transmission GratingsOne popular style of grating is the transmission grating. This type of diffraction grating is created by scratching or etching a transparent substrate with a repetitive, parallel structure. This structure creates areas where light can scatter. A sample transmission grating is shown in Figure 1. The transmission grating, shown in Figure 1, is comprised of a repetitive series of narrow-width grooves separated by distance ![]() (2) where both
Figure 2. Reflective Grating Reflective GratingsAnother very common diffractive optic is the reflective grating. A reflective grating is traditionally made by depositing a metallic coating on an optic and ruling parallel grooves in the surface. Reflective gratings can also be made of epoxy and/or plastic imprints from a master copy. In all cases, light is reflected off of the ruled surface at different angles corresponding to different orders and wavelengths. An example of a reflective grating is shown in Figure 2. Using a similar geometric setup as above, the grating equation for reflective gratings can be found: ![]() (3) where Both the reflective and transmission gratings suffer from the fact that the zeroth order mode contains no diffraction pattern and appears as a surface reflection or transmission, respectively. Solving Eq. 2 for this condition, This issue can be resolved by creating a repeating surface pattern, which produces a different surface reflection geometry. Diffraction gratings of this type are commonly referred to as blazed (or ruled) gratings. An example of this repeating surface structure is shown in Figure 3.
Blazed (Ruled) GratingsFigure 4. Blazed Grating, 0th Order Reflection Figure 3. Blazed Grating Geometry The blazed grating, also known as the echelette grating, is a specific form of reflective or transmission diffraction grating designed to produce the maximum grating efficiency in a specific diffraction order. This means that the majority of the optical power will be in the designed diffraction order while minimizing power lost to other orders (particularly the zeroth). Due to this design, a blazed grating operates at a specific wavelength, known as the blaze wavelength. The blaze wavelength is one of the three main characteristics of the blazed grating. The other two, shown in Figure 3, are The blazed grating features geometries similar to the transmission and reflection gratings discussed thus far; the incident angle ( The 0th order reflection from a blazed grating is shown in Figure 4. The incident light at angle ![]() Figure 6. Blazed Grating, Incident Light Normal to Grating Surface Figure 5. Blazed Grating, Specular Reflection from Facet The specular reflection from the blazed grating is different from the flat surface due to the surface structure, as shown in Figure 5. The specular reflection, ![]() (4) Figure 6 illustrates the specific case where ![]() (5) Littrow ConfigurationThe Littrow configuration refers to a specific geometry for blazed gratings and plays an important role in monochromators and spectrometers. It is the angle ![]() (6) ![]() Figure 7. Littrow Configuration The Littrow configuration angle, ![]() (7) It is easily observed that the wavelength dependent angular separation increases as the diffracted order increases for light of normal incidence (for ![]() (8)
where The first issue with using higher order diffraction patterns is solved by using an Echelle grating, which is a special type of ruled diffraction grating with an extremely high blaze angle and relatively low groove density. The high blaze angle is well suited for concentrating the energy in the higher order diffraction modes. The second issue is solved by using another optical element: grating, dispersive prism, or other dispersive optic, to sort the wavelengths/orders after the Echelle grating.
Figure 8. Holographic Grating Holographic Surface GratingsWhile blazed gratings offer extremely high efficiencies at the design wavelength, they suffer from periodic errors, such as ghosting, and relatively high amounts of scattered light, which could negatively affect sensitive measurements. Holographic gratings are designed specially to reduce or eliminate these errors. The drawback of holographic gratings compared to blazed gratings is reduced efficiency. Holographic gratings are made from master gratings by similar processes to the ruled grating. The master holographic gratings are typically made by exposing photosensitive material to two interfering laser beams. The interference pattern is exposed in a periodic pattern on the surface, which can then be physically or chemically treated to expose a sinusoidal surface pattern. An example of a holographic grating is shown in Figure 8. Please note that dispersion is based solely on the number of grooves per mm and not the shape of the grooves. Hence, the same grating equation can be used to calculate angles for holographic as well as ruled blazed gratings. Reflective GratingsReflective grating master copies are made by depositing a metallic coating on an optic and ruling parallel grooves in the surface. Thorlabs' reflective gratings are made of epoxy and/or plastic imprints from a master copy, in a process call replication. In all cases, light is reflected off of the ruled surface at different angles corresponding to different orders and wavelengths. All of Thorlabs' ruled reflective diffraction gratings exhibit a sawtooth profile, also known as blazed, while our reflective holographic diffraction gratings exhibit a sinusoidal profile. For more information, please refer to the Gratings Tutorial tab.
Transmission GratingsTransmission gratings are created by scratching or etching a transparent substrate with a repetitive, parallel structure. This structure creates areas where light can scatter. Thorlabs' transmission gratings are manufactured using the ruled method, which creates a sawtooth diffraction profile. Transmission gratings can also be made of epoxy and/or plastic imprints from a master copy, in a process call replication. For more information, please refer to the Gratings Tutorial tab.
Selecting a grating requires consideration of a number of factors, some of which are listed below:Efficiency: Blaze Wavelength: Stray Light: Resolving Power: For further information about gratings and selecting the grating right for your application, please visit our Gratings Tutorial. Caution:
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